isl_morph_get_dom_space: extract out isl_morph_peek_dom_space
isl_morph_get_dom_space: extract out isl_morph_peek_dom_space

This function will be reused in the next commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl